In general, an acrylic polymer used as a base polymer of an acrylic adhesive (pressure-sensitive adhesive) has been prepared by a solution polymerization of a monomer mixture including a (meth)acrylic acid ester as a main monomer component. However, due to constraints in the safety of organic solvents and environmental sanitation, emulsion polymerization and photopolymerization have been frequently employed in recent years.
Particularly, there is a problem that acrylic pressure-sensitive adhesives prepared by emulsion polymerization cannot offer equivalent or superior performance to acrylic pressure-sensitive adhesives prepared by solution polymerization. On the other hand, acrylic pressure-sensitive adhesives prepared by photopolymerization are advantageous because they are capable of offering equivalent or superior performance to acrylic pressure-sensitive adhesives prepared by solution polymerization.
Since radical polymerization of a (meth)acrylic acid ester monomer is inhibited by oxygen, it is difficult to perform the polymerization in the atmosphere in some cases. Thus, a process which includes conducting ultraviolet light polymerization (UV polymerization) in an inert gas stream such as a nitrogen gas stream has been proposed (see Patent Document 1). However, this process has a problem that a change in the monomer composition of a photopolymerizable composition caused by the inert gas stream results in variation in the pressure-sensitive adhesive properties between the front surface of the pressure-sensitive adhesive and the back surface thereof.
There has been known another process for producing a pressure-sensitive adhesive tape or sheet (hereinafter “a tape or sheet” is merely referred to “a tape” or “a sheet” in some cases) which includes preparing a monomer mixture including a (meth)acrylic acid ester as a main monomer component to have a coatable viscosity, coating the mixture to a light-permeable substrate such as a polyethylene terephthalate (PET) film, and then irradiating it with ultraviolet light (UV light) to thereby form an adhesive layer (pressure-sensitive adhesive layer) on the substrate (see Patent Document 2). Although this process is free from such a problem as described above, there arises another problem that the light permeable film (transparent film) should be peeled after the completion of the polymerization, thereby increasing in cost in case of discarding the film after one use. Therefore, reusing such a peeled film has been considered by definition.
Even though the monomer mixture layer (pressure-sensitive adhesive composition layer) as described above is sandwiched between films, the side surfaces thereof are exposed to the atmosphere. Although it causes no problem in case of a thin monomer mixture layer, the area being in contact with oxygen in the atmosphere is enlarged with an increase in the thickness of the monomer mixture layer as described above. As a result, there arises a problem that the polymerization degree in the side surfaces (side edge parts) of the above-described monomer mixture layer is lowered at the photopolymerization (photo-curing) due to the polymerization inhibition by atmospheric oxygen and thus the cohesive force in the side surface of the thus formed pressure-sensitive adhesive layer is lowered. Accordingly, when a film is peeled off from the pressure-sensitive adhesive layer, a part of the side surface of the pressure-sensitive adhesive layer remains on the film, thereby being impossible to reuse the film.                Patent Document 1: JP-A-3-285975        Patent Document 2: U.S. Pat. No. 4,181,752        